SummaryEsterase activity was determined in 723 pig‐serum samples with a‐naphthyl acetate as substrate, and in 591 samples with β‐naphthyl acetate. The activities showed a wide range and were distributed in groups. β‐naphthyl acetate gave a specific reaction to arylesterase and spread the activities in at least 17 groups. Yorkshire pigs had in general a higher level of arylesterase activity than Swedish Landrace pigs. The results were in agreement with the theory of multiple alleles controlling the arylesterase activities (Augustinsson and Olsson, 1961). In addition, the cholinesterase activity showed a systematic variation. No differences in the migration rates of the arylesterase zones could be detected in starch‐gel elec